edm::refToPtr now handles null Refs

Previous, edm::refToPtr would cause an exception to happen if it was passed a ‘null’ edm::Ref. This change now just returns a null edm::Ptr in such a case.
Merge pull request #2049 from Dr15Jones/dealWithNullRefsInConversionToPtr

Framework fixes -- edm::refToPtr now handles null Refs